1. Field of the Invention
The present invention relates to a CMOS image sensor which includes a photodiode and an MOS transistor on a common substrate, and to a manufacturing method thereof.
2. Description of the Related Art
Conventionally, CCDs (charge coupled devices) are known as solid imaging devices for converting image signals into electric signals. A CCD comprises a photodiode array, and pulsed voltage is applied to the charge stored by each photodiode, so as to output a corresponding electric signal. Recently, CMOS image sensors, having a chip form which includes a photodiode and an MOS transistor, are used as solid imaging devices. A CMOS image sensor can be easily manufactured and the chip size thereof can be reduced.
In order to increase the operation speed of the MOS transistor for converting the charge generated by the photodiode into an electric signal, Japanese Unexamined Patent Application, First Publication, No. 2000-31449 discloses a technique of forming a silicide film in a high-concentration diffusion area of the CMOS transistor. This silicide film is a compound of Si and a refractory metal such as Ti, Co, or the like, and the silicide film can reduce resistance of the diffusing layer or the gate electrode of the transistor (i.e., parasitic resistance), thereby improving the operation speed and reducing the operation voltage.
However, in a conventional CMOS image sensor, incident light may be reflected by a surface of the photodiode (i.e., light reception surface), which degrades the sensitivity. This is because the surface of the photodiode is covered by only an oxide film and incident light is reflected by the light reception surface, thereby reducing the amount of incident light and also reducing the amount of the electric charge after the photoelectric conversion. In order to prevent such reduction, an antireflection film may be provided on the light reception surface. However, in this case, an extra lithography process is necessary, and thus the manufacturing cost increases.
Additionally, in order to improve the operational characteristics of the CMOS transistor used in the CMOS image sensor, it is necessary to form a silicide layer in a diffusion layer, as in the conventional technique. However, when a silicide layer is formed in a diffusion layer of a photodiode, the sensitivity characteristics of the photodiode degrade. In order to prevent such degradation, a specific lithography process is necessary, which also increases the manufacturing cost.
In consideration of the above circumstances, an object of the present invention is to provide a CMOS image sensor and a manufacturing method thereof, for improving the sensitivity characteristics of the photodiode and the operation characteristics of the MOS transistor, and realizing easy and low-cost manufacturing.
Therefore, the present invention provides a CMOS image sensor comprising:
a photodiode having a light reception surface covered by a multi-layered antireflection film which is formed by alternately depositing two or more kinds of insulating films whose refractive indexes are different; and
at least one MOS transistor having diffusion layers which respectively function as source and drain areas, wherein a silicide layer is formed on each diffusion layer, and
wherein the photodiode and said at least one MOS transistor are provided on a common substrate and are electrically connected with each other.
Said at least one MOS transistor may have a gate electrode which has side walls.
The present invention also provides a method of manufacturing a CMOS image sensor, comprising the steps of:
forming a multi-layered antireflection film by alternately depositing two or more kinds of insulating films whose refractive indexes are different, on an area including the surfaces of a photodiode and an MOS transistor provided on a common substrate;
patterning the multi-layered antireflection film in a manner such that diffusion layers which will respectively function as source and drain areas of the MOS transistor are exposed and the multi-layered antireflection film remains on the light reception surface of the photodiode;
forming a refractory metal layer on an area which includes the surfaces of the diffusion layers and the remaining multi-layered antireflection film, wherein the metal layer is provided for forming a silicide layer;
forming a silicide layer on the diffusion layers by thermal-processing the common substrate including the metal layer; and
removing the remaining portion of the metal layer, which remains after the thermal processing performed in the previous step.
The method may have a step of forming side walls at either side of a gate electrode of the MOS transistor after the multi-layered antireflection film is patterned.
According to the CMOS image sensor having a photodiode whose light reception surface is covered with a multi-layered antireflection film, the reflectance can be reduced within a broad wavelength range and the sensitivity characteristics of the photodiode can be improved. In addition, a silicide layer is formed on each surface of the diffusion layers which function as the source and drain areas, thereby improving the operation speed and decreasing the operation voltage. These effects can be obtained by the CMOS image sensor having a single chip form.
According to the above method, a CMOS image sensor having the above-explained characteristics can be easily manufactured with low cost. That is, in a process of making the silicide reaction occur in the surface of a diffusion layer of the MOS transistor, the insulating multi-layered antireflection film also functions as a mask for preventing a silicide layer from being generated on the light reception surface of the photodiode. Therefore, no extra mask-forming process is necessary.
In addition, in a process of forming the side walls at either side of the gate electrode of the MOS transistor, the insulating multi-layered antireflection film also functions as a mask for preventing an etch-back effect (occurring when the side walls are formed) from affecting the light reception surface of the photodiode. Therefore, no extra mask-forming process is necessary also in this case. Accordingly, a photodiode having superior sensitivity characteristics can be easily manufactured.